The polymerisation of monomer in both gas phase and slurry processes is well-known and widely operated commercially. In either case monomer, and optionally comonomer, are polymerised in the presence of a polymerisation catalyst to form the polymer.
The polymerisation reaction is highly exothermic. It is therefore important to maintain efficient cooling throughout the reaction zone, and to prevent hot-spots. This is generally achieved by maintaining strong circulation of reactants e.g. in a fluidised bed or slurry loop, and trying to avoid stagnant spots.
However, many of the connections to the reactor for feeding and withdrawing materials, and for process monitoring or control, form zones which have the potential for stagnant reaction medium. In particular, most connections comprise a valve and a line by which the valve is connected to the reactor. When the valve is closed the line between the valve and the reactor forms a zone with the potential for stagnant reaction medium. Reaction in these zones can form polymer, which may foul or block the line. Some of these connections may be linked to critical services, such as compressor seals or relief valves, which lose their effectiveness if exposed to the polymer/polymerisation reaction.
For this reason it is known to flush such zones both to prevent build-up of polymer in the zone and also to prevent contact of the polymerisation mixture with seals and the like.
This is usually done using a gas or liquid which is not harmful to the materials, such as seals, in the zone, and also which is not overly detrimental to the polymerisation reaction. Examples of suitable materials in a polymerisation process can include inert gases such as nitrogen or inert liquids such as slurry reaction diluent.
Whilst nitrogen is a useful flush gas, in that it is inert and usually readily available, an issue is that, although it does not directly affect the polymerisation, being inert it can build-up to relatively high levels, and thus needs to be purged from the system. Being of a similar molecular weight to ethylene, however, the purge often leads to losses of significant quantities of valuable materials.